Epoxy resin compositions are used in a wide variety of fields, such as adhesion, casting, encapsulation, lamination, molding and coating, due to their excellent cured properties and ease of handling. There are a large variety of epoxy resins with greatly varying cured properties. Therefore, epoxy resins should be chosen according to their intended purpose.
In recent years, the requirements for various characteristics of polymeric materials have become more stringent as conditions under which the polymeric materials are used have become more severe. Many commonly used epoxy resins have been unable to accommodate these increased demands.
For example, epoxy resin compositions are used for semiconductor encapsulation. However, the required performance level has increased in this field. As high integration of semiconductor devices has increased, the resulting enlargement of semiconductor elements has been remarkable, creating a need for miniaturized and thinned packages. In addition, mounting of semiconductor devices has shifted to surface mounting. In surface mounting, the semiconductor devices are directly dipped into a solder bath, thus being suddenly exposed to high temperature. Due to rapid expansion of absorbed water, high stress is applied to the whole package, generating cracks in the encapsulating material. For this reason, epoxy resin compositions for encapsulation having good solder crack resistance require high heat resistance (high glass transition temperature), low moisture absorption and low stress property (a small thermal expansion coefficient).
To reduce moisture absorption and improve resistance to thermal stress, a large amount of an inorganic filler such as fused silica powder is commonly used. This significantly improves solder crack resistance. However, if an excessive amount of the inorganic filler is used, fluidity at molding is impaired. For this reason, an epoxy resin for semiconductor encapsulation is required to have a low melt viscosity.
In addition, for use with thinned and miniaturized packages, the epoxy resin for semiconductor encapsulation is also required to have high fluidity, thus the demand for low melt viscosity of the epoxy resin is even greater.
Further, rapid curability is required in order to increase molding speed, and it is the recent trend that materials not having rapid curability are not used even if other characteristics are good.
Novolak type epoxy resins (particularly cresol novolak type epoxy resin) currently in common use have excellent heat resistance and rapid curability, but have been unable to sufficiently meet low moisture absorption and low melt viscosity requirements.
Further, phenol novolak resin currently in common use as an epoxy resin hardener is also insufficient with respect to low moisture absorption and low melt viscosity, although heat resistance and curability properties are excellent.
The use of biphenyl type epoxy resins having a low melt viscosity has been widely investigated. Japanese Patent Application No. Sho 61-47725, for example, describes mixing 20-400 parts by weight of a novolak type epoxy resin per 100 parts by weight of a biphenyl type epoxy resin, and using phenol novolak resin as a hardener, to take up the balance. However, because the heat resistance of these compounds is poor, they are not sufficiently solder crack resistant. Curability and moisture absorption properties of these resins are also insufficient.
Japanese Patent Application No. Hei 7-118366 and Japanese Patent Application No. Hei 7-216054 describe improved solder crack resistance by mixing a novolak type epoxy resin and 50 wt % or more of a tetramethyl biphenyl type epoxy resin in addition to an aralkyl phenol resin which achieves low moisture absorption, as a hardener. However, if tetramethyl biphenyl type epoxy resin, which has poor curability, is added in a large amount, curability is insufficient.